The present invention relates to ice cube trays used for household refrigerators and is especially related to the ones with a double structure of inner and outer trays enabling transparent ice to be made.
An example of a conventional ice cube tray of this known type is the one disclosed in Japanese Provisional Utility Model Publication No. 185073/1988, which consists of an inner tray divided lengthwise and breadthwise into plural numbers of cubic pockets with an open top, and of an outer tray divided into such configuration that fits the shape of said inner tray, with a space to be formed, when assembled, between the outer bottom of the inner tray and the inner bottom of the outer tray, with some openings provided on the bottom of the inner tray to adjoin the said space so as to make transparent ice in the said cubic pockets.
The water in an ice cube tray freezes first from the surface and then downward to the bottom. Since it starts freezing first from pure water, the end result is that opaque ice containing air and impurities such as chlorinated lime is intensively made in the bottom area of the ice cube tray. The ice cube tray of this kind is so designed as to make use of this phenomenon, i.e., getting only the transparent ice made in the inner tray by driving the opaque part away into the said space in the lower part of the ice cube tray.
Another example of an ice cube tray, as seen in Japanese Official Utility Model Publication No. 26864/1992, is the one with an upper space provided between the inner and the outer trays so as to absorb expansion of the freezing water. However, in this form of an ice cube tray, the said upper space is provided only around the whole body of the ice tray, so that the expansion caused in the central area of the ice cube pockets cannot be completely absorbed, thus making it difficult to get transparent ice in every ice cube pocket especially when the ice cube tray has a large number of ice cube pockets.
Another problem was that the thick ice layer formed between the inner and the outer trays makes both of the trays stick together tightly, thus making it difficult to release the ice cubes by twisting the tray, which can be easily done in the case of a general ice cube tray without a double structure.